The present invention relates to a speed controller for controlling a DC motor to run at a constant speed, substantially irrespective of the level of load applied to the motor within a specific range of motor loads.
In the prior art, a method of DC motor speed control is known whereby an electrical generator unit, known as a tachogenerator, is attached to the body of the DC motor. This tachogenerator is employed to sense the speed of rotation of the motor, with an output voltage produced from the tachogenerator being compared with a reference voltage. The supply voltage applied to the DC motor is controlled in accordance with the result of this comparison to compensate for changes in motor speed.
With another prior art method of speed control, the power supply terminals of the DC motor are connected in one arm of a bridge circuit, and the counter electromotive force produced by the motor during rotation is sensed and compared with a reference voltage level. The result of this comparison is used to control the voltage supplied to the motor at a constant value.
One disadvantage of the first of the above prior art methods is that the axial length of the motor tends to increase. In addition, the overall control apparatus required is complex. In the case of the second prior art method, in which the DC motor is connected in one arm of a bridge circuit, a resistor is connected in series with the motor to impede high motor current with the result that a substantial amount of heat is wasted.